1. Field of the Invention
This invention relates to a process for preparing unsaturated carboxylic acids from unsaturated aldehydes in the presence of a phosphorus - molybdenum - arsenic - alkali metal type catalyst which contains two or more other metals.
2. Description of the Prior Art
Various catalysts have been known for use in the gas phase catalytic oxidation of unsaturated aldehydes such as those disclosed in U.S. Pat. Nos. 3,475,488; 3,567,773; 3,646,127; 3,649,684; 3,686,294 and 3,761,516 and DT 2,251,364. Included among these catalysts are molybdenum - vanadium catalysts which exhibit excellent results in the oxidation of acrolein. However, these catalysts are not suitable for the oxidation of methacrolein. Phosphorus - molybdenum - arsenic catalysts of specific compositions are known to give good results in the oxidation of methacrolein. However, these catalysts are still insufficient because several problems still remain to be solved. For instance these catalysts have a very short lifetime. Other types of catalysts do not give good results in the oxidation of methacrolein. A need, therefore, continues to exist for a catalyst for the oxidation of unsaturated aldehydes such as acrolein and methacrolein to the corresponding unsaturated acids which exhibits excellent catalytic effects and which has a good lifetime.